In preparation for this meeting with Daisy, Gatsby has attempted to create the perfect environment. He is a master of controlling the world around him and has obtained nearly everything he has ever wanted through his industrious labor and insights. One thing has eluded him thus far: Daisy. On this day, he dresses to impress and makes sure that the grass looks perfect, sending his own gardener to Nick's house to tidy things up. He even sends a "greenhouse" of flowers to decorate Nick's home.
On a day when Gatsby is supposed to be reunited with the love of his life, you would expect a bright, sunny day, full of warmth. Yet as Gatsby arrives, it is "pouring rain." This is a reminder that Gatsby is not in complete control of this meeting with Daisy. It casts a sense of gloom over the meeting, particularly as he becomes more convinced that Daisy won't show up. Finally the rain begins to pass just before Daisy's much-anticipated arrival. Her presence is juxtaposed to the rainy and gloomy mood thus far:
Under the dripping bare lilac trees a large open car was coming up the drive. It stopped. Daisy's face, tipped sideways beneath a three-cornered lavender hat, looked out at me with a bright ecstatic smile.
Though the rain has reflected Gatsby's pervasive sense of melancholy in awaiting this arrival, Daisy's "bright ecstatic smile" lifts this sense of gloom even as the trees continue to drip the remnants of rain behind her. This indicates the power Daisy has over Gatsby and the influence she has on his emotional state.
Daisy and Gatsby begin getting reacquainted inside the house, and Nick quietly excuses himself to give them some privacy. Once outside, he notices that the weather has shifted:
After half an hour the sun shone again.
The day has become clear and bright, reflecting the way Gatsby's spirits have improved upon Daisy's arrival. He believes that this moment is the culmination of years of determination and that his tormented longing for the girl of his dreams is over.